Method of manufacturing sintered



United States Patent 9 METHOD OF MANUFACTURING SINTERED CATHODE RobertT. Lynch, Berkeley Heights, N .J., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation of New York NDrawing. Application May 11, 1956 Serial No. 584,180

7 Claims. (Cl. 75-207) novel method of producing cathodes for thermionictubes comprising the steps of mixing nickel powder with barium andstrontium carbonates, molding under pressure and firing. Although theelements therein described showed promise as compared with the moreconventional type of sprayed cathode element, thermionic tubes utilizingsuch cathode elements have not appeared on the market and do not appearto have been used commercially.

There is herein described and claimed an improved process for themanufacture of molded thermionic cathode elements of the type describedin the above-designated United States patent. Briefly, the process ofthis invention comprises mixing nickel powder with a powdered materialcontaining a barium compound such as powdered double coprecipitatedbarium-strontium carbonates together with an activating agent and abinder, molding the mixture under pressure into the desired shape andheating and cooling in the presence of a succession of atmosphere gasesof different compositions which perform varying functions over selectedtemperature ranges.

Cathode elements so prepared have been in experimental use for some timeand have manifested certain properties, notably hardness and smoothness,which are superior to commercially available structures.

Molded cathode elements prepared in accordance with this invention areless susceptible to harmful atmospheres such as air than are theirsprayed counterparts. In ex' periments conducted by the inventor,cathode'elements have actually been removed from their envelopes andexposed to air for long periods of time without peeling or loss ofcoating, and with no appreciable loss of emission. They have withstoodthe drawing of large 'D.'C.

current of several amperes per square centimeter with,

no fading.

Molded elements operated under pulsed conditionssuch as to createsparking repeatedly have shown no electrical deterioration, of thepulsed or DC. emission characteristics or physical deterioration of thecoated surface. Due, in part, to the increased thickness of the emittingsurface,

these elements are less susceptible to ion bombardment than are sprayedoxide cathodes.

Molded thermionic cathode elements may be easily. They may bemanufactured on a commercial scale. I pressed in any of a large varietyof shapes, limited only by die techniques. For example, they may bepressed into; concave gun-type cathodes, they may be formed intocomposite structures as by pressing together multiple layers of a puremetal powder such as nickel and emitting layers heater structures byembedding and pressing .the heater "ice material into the emittingmixture. Other structures will suggest themselves to those skilled inthe art.

, Automation presents no problem. After the prepara: tion of the initialmixture, all steps may be carried out automatically with no undueprecautions being taken regarding cleanliness. Subsequent to pressingand heat, treatment, the elements may be easily machined so as toproduce any desired configuration.

A general outline of the methods herein together with the ranges ofoperating parameters will now be given.

An initial emitting mixture is prepared. This mixture contains nickelpowder, an emitting material such as coprecipitated barium-strontiumcarbonate, an activating agent, sometimes referred to as an activatingagent, .and a binder material which latter will be removed during thesubsequent heating steps. The grade of nickel powder chosen should be asnearly pure as practical so as not to contain any contaminant which mayimpair the emitting characteristics of the final structure.Carbonyl-nickel powder has been found suitable in this use. Electrolyticnickel powder may be substituted. Although the particle size of thenickel powder is not critical in most uses, a general preference existsfor very fine particles. It has been found that 100-mesh materialcontaining particles of a maximum size of 150 microns producessatisfactory results. In the production of cathodes for use inmicrooscilloscope tubes Where a very fine uniform surface is required,particles as small as 4 microns have been used with an accompanyingimprovement in characteristics as compared with the coarser material. W

Any of the powdered emitting mixtures well known in the preparation ofsprayed thermionic cathodes may be used in the preparation of themoldedcathode. materials usually contain a barium compound, which will,break down on station to yield barium oxide. Since the temperatureattained on station is usually about 1000" Q, for the purpose of theprocess described herein, it is. considered that any barium compoundwhich will thermal-- ly decompose at a temperature of less than 1000 C.to yield barium oxide is suitable. Such materials include the singlecarbonate material, barium carbonate; the double carbonate material,coprecipitated barium-stumtium carbonate; and the triple carbonatematerial, coprecipitated barium-strontium-calcium carbonate. In general,it has been found that the double carbonate is to be preferred over thesingle and that little further advantage is gained by use of the triplecarbonate.

The double carbonate most commonly available for this pur-E pose is acoprecipitant of equimolar portions of barium carbonate and strontiumcarbonate. The particle size.

. of this emitting mixture is not critical, a preference existing againfor fine particles. A commercially availablei coprecipitant containingparticles, percent of which.

are smaller thanlO microns, has proved suitable.

Activators which perform the function of producing the emissioncharacteristics of the structure are wellL known in the sprayed cathodeart and reference may be six thousand hours of use. Other activatorsinclude carbon which is even more rapid than zirconium, and siliconwhich has some of the characteristics of titanium but which may developan interface of silicon dioxide which impairs the operatingcharacteristics of the resultant device. Ma-gnesiu-m has been tried andfound to be un's'atis- I factory in this use, it being found to be tooreactivean d resulting in the production of excessive amounts ofmagnesium oxide which prevents further activation. Depending on the useto which the final structure is to be put, of the activators mentioned,either zirconium or titanium, is to be preferred. Whichever activatormaterial is used, it should be powdered as finely as is feasible, anaverage particle size of 15 microns being found satisfactory.

The preceding paragraph is not intended to be an exhaustive discourse onthe general subject of activator ma terials. Such materials and theircharacteristics are wellknown to those skilled in the art. See, forexample, Theoretical Study of the Chemistry of Oxide-Coated Cathodes, E.S. Rittner, Phillips Research Reports, vol. 8, page 184, 1953. It iswell known that activator materials such as zirconium and titanium areconveniently used in a chemical form which breaks down to give a finedispersion during processing. It is quite common in the instance ofthese elements to use the corresponding hydrides. Activator propertiesindicating the selection of one or another of the materials included inthis broad grouping are also well known.

. In addition to the nickel powder, the carbonate, and the activatormaterials listed above, there may be added to the first mixture a bindermaterial. Binder materials which may perform the second additionalfunction of acting as a lubricant are well known to those skilled in related fields such as, for example, ferrite art. It is a generalrequirement of such materials that they leave little or no residue inthe end product after sintering. Common binder materials which willoperate satisfactorily here include acetone solutions of eitherisobutylmethacrylate or stearic acid. For other common binders andassociated characteristics, see Treatise on Powder Metallurgy," Goetzel,vol. 2, Interscience Publishers, Inc., New York, 1949. Binders should beadded to the mixture in minimum quantities. Where excessive amounts arepresent, resultant diificulties include porosity and excessiveflexibility of final product, possible contamination due to impuritieswhich may be contained in the binder, and difficulty of removal. Whereit is undesirable to use a binder as, for example, when the cathode issintered in vacuum, the die plungers may be lubricated with paraffin.

The following is an outline of the procedure to be followed in producinga cathode element from the above materials.

To the nickel powder there is added from zero to 2 percent by weight ofan activator material. For most uses the preferred amount of activatoris of the order of 1 percent by weight of the nickel powder. Use ofamounts of activator in excess of about 2 percent results in a fallingoff of the activity of the end product. The mixture of nickel powder andactivator is thoroughly dryrnixed as, for example, in a mortar andpestle or in a ball mill. Experience indicates that a mixture of about20 or 30 grams may be thoroughly mixed in a mortar and pestle in lessthan 15 minutes. This mixing step is carried out in air at roomtemperature.

The acetone solution of binder is produced by dissolving from 1 to 2percent of binder in the acetone in air at room temperature. Althoughheating will hasten the formation of this solution, it is to be avoidedunless proper precautions are taken to prevent fire.

A binder solution in an amount of up to about 2 percent by weight ofnickel powder is slowly added to the nickel powder-activator mix in amortar at such a rate as to maintain a slurry. Mixing is continued witha pestle as the binder solution is added in air at room temperatureuntil the mixture is dry, the acetone evaporating as the binder isadded. Again, by reason of the flammability of acetone, this mixing stepis carried out in an unheated mortar unless additional precautions aretaken.

, This mixture is herein referred to as the nickel mix may be storeduntil required.

, A second basic mixture is now produced by mixing a portion of thenickel mix above with a portion of single, double or triple carbonate.The amount of carbonate used represents a compromise between pure nickelmix which is best from a mechanical standpoint and pure carbonate whichis best for emission. The amount of carbonate is generally in the rangeof from about 10 percent to about 50 percent by weight of nickel mix,the preferred amount for pressed cathodes having a supporting portion ofmetallic nickel being about 30 percent by weight. Mixing is carried outin a mortar and pestle or ball mill and is continued until the color ishomogeneous. Since the nickel mix is black and the carbonate is anolf-white, this final mixture will be gray. With a total amount of about30 grams, mixing in a mortar and pestle will take about 15 minutes. Thisfinal mixture is herein referred to as the emitting mix.

The nickel mix and the emitting mix having been produced, the next stepin the process is to press the materials into the desired shape andsize. If the final product is to be a composite structure, layers of thenickel mix and the emitting mix may be pressed in one operation. In sucha structure, the nickel layer lends mechanical rigidity while theemitting mix layer may be kept relatively shallow so as tokeep to aminimum the time on station. On the other hand, the final structure mayconsist only of emitting material where, for example, it is to beproduced by pressing or otherwise adhering emitting material to anexisting structure.

The usual procedure, where the structure is to be composite, is to firstinsert a layer of nickel mix into a die and after pressing this layerlightly, to then insert a layer of emitting mix into the die. Theentirety, consisting of the two layers, is then pressed at a pressure offrom 20 tons per square inch to 100 tons per square inch. It has beenfound that a pressure of about tons per square inch, readily availableon commercial hydraulic presses, is suitable in producing a dense masswhich may be easily machined. The greater the applied pressure the moredense is the end product and the greater is the initial activity.However, increasing the applied pressure beyond about tons per squareinch results in a physical breakdown of the emitting surface.

There are several considerations to be taken into account in determiningthe optimum shape and size of the pressed body as produced in accordancewith the steps outlined above. In general, if the emitting layer, thatis the layer produced by firing of the emitting mix portion, is toothick, there is difiiculty in removal of gas which is evolved uponbreakdown of the carbonates partially during sintering but primarily onstation. In addition, little is gained by increased thickness since inthe usual tube configuration all useful emission evolves from the planarsurface or other surface facing the anode. Side emission from surfacesof the cathode not facing the anode does not significantly increase thecurrent density of such a structure. In general, emitting layerthicknesses of the order of from about 3 to about 8 mils are to bepreferred, this range being sufi'iciently thin to avoid undue difficultyof removal of evolved gases and being sufficiently thick to produce adurable emitting surface which will withstand any expected arcing,bombardment or exposure to harmful atmosphere such as air. If thethickness of the emitting layer is too thin, as below about 3 mils, adiscontinuity in the emitting surface may result. Such a discontinuityis undesirable in that the current density of the tube is decreased andother operating characteristics may be impaired. For compositestructures as, for example, those utilizing a layer derived from anemitting mix and a layer derived from a nickel mix, the optimum emittinglayer thickness may be of the order of 5 mils.

For composite structures the thickness of the non-emit- C ting layer isdetermined primarily with a view to mechanical considerations. It hasbeen found that with an emitting layerof the .order of about 5 mils inthickness, at

Once the structure has been pressed, whether it consists of a singleemitting layer or is a composite structure; it is now subjected to heattreatment. The chief purpose ofthis heat treatment is to sinter thenickel in the emitting mix so as to produce. a mechanically rigid body.This heat treatment step is most critical and precautions must be takento avoid contamination, to avoid undue oxidation of nickel so as toachieve good sintering and to avoid the reduction of the alkaline earthcarbonates to the oxides which will, react with the atmosphere toproduce hydroxides. It is considered that the development ofthe'following series of heat treatment steps constitutes the Any inert gassuch as. helium or argon may be substituted for the; nitrogen providingits impurity contentis not un-' desirable.

The nitrogen flow is then replaced by a flow of from,

225 to 350 cubic centimeters per second of purified d'r-y hydrogen orprepurified hydrogen (PPH). The exit .hydrogen is burned in a pilot atthe exit. end of the furnace.

After the exit hydrogen has been ignited the furnace is put intooperation and is heated from room temperature to about 600 C. at a rateof about 100 C. per minute. The purpose of the hydrogen flow. is toprevent any substantial oxidation of the nickel particles in theemittingand to reduce any nickel oxide which may be present-Heating'over this range also has the effect of breaking down a smallamountof the carbonate present to oxides a; consequent release of carbondioxide. If the fur-[. nace-is heated at a substantiallyzgreater ratethan 100 C.

per minute, the released gases, the oxygenvfrom the nickeloxide and thecarbon dioxide'from the carbonates, may cause eruption and destroy thehomogeneity of the pressed body. In general; a slower heating rateduring. the hydrogen flow period isnot objectionable, although reducingtoa very low rate as, for example, below .the rateof L0.

C. per minutemay result in breakdown of larger arnountsl' of carbonate.

When the temperature of the furnacereaches 600 C. it'is held at thattemperature as the gas flow is'changed from hydrogen to nitrogen orotherinert gas as helium orargon until the exit pilot flame becomesextinguished.

I This. gen-- erally takes about from 1 to Z'minutes. The flow rateis.

indicating a removal of residual hydrogen.

not critical, but as in'purging, a. flow rate of about 50.

cubic centimeters per second of nitrogen has been found.

sufficient.

Although they temperature at. which the changeover from i hydrogen tonitrogen is carried out is' generally held at about 600 C., it has beenfound that this changeover may."

satisfactorily be carried out over the range of from 5503' am 650 C.Changeover below 550 C. results in in.- sulficient reduction-of totaloxide intthe nickel eventually. resulting. in imperfect, sintering,while the presence of a. hydrogenatmosphere at a temperature of over 650C. is undesirable for the reason that too great an amountof thecarbonatesare-reduced to the oxides.

Once the exit pilot is extinguished, the temperaturerofthe. furnace isagain caused to rise, this time to a tempera- A nitrogen flow rate ofthe order of about 50 ture of at least 800 C. During this lastheatingstepthe nickel powdersi'nters, substantial sintering. takingplaceat a temperature of about 800 C. A sintering temperature of about1000 C. is usually. preferred. Temperatures above about 1200" C. areunsatisfactory in that larger amounts of carbonate breaks down.

When the temperature of the furnace attains thedesired sinteringtemperature, .the power is turned 0E and the fur nace allowed to cool toabout 600 .C. Nitrogen flow: through the furnace is maintained duringthis cooling steps In general, the coolingrate is not critical providingthat therate is not such'as. to produce serious thermal stress andresultant cracking .of the cathode. Maintenance of: the furnace at anytemperature in the: range ofover 800. C. results in increased breakdown.of the carbonates and should be keptat a minimum.

When. the furnace has cooledto about 600 C. the? nitrogen flow isstopped and hydrogen is caused to flow through the furnace, the exhaustpilot again being; lighted to prevent formation of a combustiblemixture; outside of the furnace. In this connection, it is againimportant that, in the temperature range below about 550 C., a reducingatmosphere be maintained within the furnace to remove any reducibleoxides which may have formed during the sintering procedure in thehigher temperature range.

The furnace is now allowed to cool to room tempera*: tur'e. Although therate of cooling is not important,- it is desirable to cool. rapidly toprevent unnecessary. contaminationof thesintered material. When thefurnace is at room temperature the flow of hydrogen. is stopped and thefurnace is purged with nitrogen or other. inert gas until: the flame isextinguished. If it is con--- sidered desirable, there is no objectionto substituting nitrogen for hydrogen inv the heating or cooling rangebetween room temperature and a temperature in: the range of 300 C. to400 C.- since the hydrogen haslittle reducing, action below about 400 C.r

With the processv carried out as set forth above, it is possible toproduce a sintered product in which no more than about 5 percent of the'total carbonate is decom-- posed to the. oxide- The .sintered cathodeelement may now be machined such-is desired, after which it may either.be placed directly in the vacuum tube structure or may be stored; invacuumuntil required.

All: thatremains in the manufacture of a usable" cathode: is :theconventional breakdown procedure; Since".

this'procedure is'well' known to those skilled in the art;

it will not be described in detail. In brief, a typical":breakdownprocedure consists of sealing the element on a vacuum stationwhich is evacuated to a pressure of the order of'10- millimeters ofmercury. The cathode istihen heated at a maximum pressure of 10millimeters until the carbonates are broken down to oxides. This heatingprocedure, which may takeof the order of 15-30 minutes for an emittinglayer thickness of approximately 5 mils, is terminated whensubstantially all of the car :xbonates are broken down. The breakdownpoint is indicated by a sudden drop in pressure within the chain-i her.The cathode is then heated to about 1000 C. and is held at thistemperature for about 5 minutes. The maximum'expected operatinganode-potential is then'ap plied:.with the structure at 1000 C. andemission current isdrawn for a period of 5-10 minutes The tent-fperature of the structure is then dropped to about 8509" C. where thetotal current is then measured by DC.

or pulse measurement.

Me'asurementsat this stage on station at .an operating temperature of850 C. indicated pulse current emis sionintensities of the order of-3 to5 amperes per: square centimeter. and D.C. emission current intensitiesofthe: order-of 1. to. 2 amperes per square centimeter...

states I The following numerical example relates to, the preps-- Iarationofamoldedcathodm Example I A 0.2 grarn of. zirconiumhydridepowder of an average particle size of microns was added to grams ofcarbonyl nickel. powder of. an averageparticle size of 1 150 micronsandof a chemical purity of 999 percent by weight, disregarding oxygen.The. combined powders were thoroughly mixed in a 4-inch mortar andpestle I Q for 10 minutes. I To this there was slowly added20 cubic Icentimeters of acetone in which .was .dissolved 0.4 gram isobutylmethacrylate. asmixingis continued. The rate of addition of theisobutyl-methacrylate solutionto the powder mixture was such as'to' atall times maintain a slurry in the mortar,. Addition time was about. 15I minutes. I Subsequent to addition,- the powders were mixed until allof the Iacetone'was' evaporated. Mixing time was about. minutes. Thismaterial will be referred to as the nickel mixfi I The emitting mix wasformed by dry mixing 3 grams I I of alkaline earth double carbonate(coprecipitated barium I strontium carbonate), in a mortar and pestlewith 7 grams I of nickel mix prepared as'above'. Mixing time was 15: Iminuteaf I I I A three-piece, double-actingdie of circular cross-section havinga 0.116-inch inside diameter holeandtwo sliding plungers was.used to mold the nickel and emitting mix into a pressed composite bodyas follows: .The lowerlplunger' of the die was inserted in the die bodywas to leave a space of a depth of 0.1 inch. The

' space was filled with nickel mix, the diewas tapped gently so as tosettle the powder and the excesspowder was.

removed. The. nickel mix in the die was thendepressed ,by .useof theupper plunger or spacer; so. as .to leave I a space of a depth of 0.015inch; The said space was then. filled .with emitting mix materialpreparedas above, the emitting mixmaterial wasjlevelecl oil at the topof v the die and the top plunger was inserted into thedie,

I body. The plungers Werethen centered inthe die body, the completeassembly was placed in a hydraulic press and a pressure of 80 tons persquare inch was .applied between plungers. The plunger and pressed discwere then ejected from the die body. The pressed disc was then placed ina nickel boat and the boat was inserted in a Globar furnace having a oneand one-half inch inside diameter. With the furnace at room temperature,it was purged by passing nitrogen gas of a grade known as prepurifiednitrogen containing no more than 0.1 percent impurities by volume at aflow rate of cubic centimeters per second for a period of 5 minutes.After the purging period had terminated, the nitrogen gas flow wasreplaced by a 25 0 cubic centimeter per second flow of pure dry hydrogenof a grade known as high purity containing as impurities no more than0.3 percent by volume. burned oil? at a pilot at the exit end of thefurnace.

The furnace was then switched on and allowed to heat at a rate of 100 C.per minute to a temperature of 600 C. The flow of hydrogen gas was thenreplaced by a 50 cubic centimeter per second flow of pure dry nitrogenof the grade utilized in initial purging. With nitrogen flowing through,the furnace was maintained-at 600 C. for one minute at which time thehydrogen flame was extinguished indicating substantial purging .ofhydrogen from the system. After extinction of the pilot, the furnace wasagain allowed to heat, this time at a rate of 250 degrees per minuteuntil a temperature of 1000 C. was attained. Nitrogen flow wasmaintained through the furnace during the entire heating period from 600C. to 1000 C. The furnace was allowed to reach a momentary peaktemperature of 1000 C. after which it was allowed to cool to atemperature of 600 C. while maintaining the nitrogen flow through thefurnace as above setforth. Cooling wasaccomplished by turning Thehydrogen was ignited andbe the power source to the furnace and took ,I6n'iiuutes. Thenitr'ogen howwas thenzreplaced by hydrogenflow of thegrade and flow rate set forth above as utilized during the initialheating procedure while maintaining j the furnace at 600 C. Theexit'fiow of. hydrogen was again ignited at'the exitpilot and thefurnace was allowed I to cool to room: temperature. I

to room temperature .took minutes. peraturethe hydrogen withinthefurnace waspurg'e-d by i I a nitrogen flow of the same grade and'flowrate as above "set forth in connectionwith initial purging; When the Iexit pilot was extinguished, indicating substantial purging I of thesystem, the boat was removed, from the furnace. 1 .The'stored'discs werethen utilized in the preparation ofthecathode. structurein a travelingwave oscilloscope I tubeas described by I. R. Piercein Electronics ofNoe At room temvernber 1949 at pages 97-99, although they could havebeen stored in evacuated glass envelopes for any desired I period. I

The cathode elements so produced were welded to or 0.500 inch by spot,welding; The cylinder was sup- I I Thecornplete oscilloscope tube wasthen sealed to a I vacuum system in which a vacuum of from 5 10- to l 3I I I portedby; a. ceramic insulator and the structure mounted I in acathode ray tube electron gun assembly as described in the above-citedreference.

5 10-T millimeter of mercury was maintained and in which the structure}was baked for 14 hours at 420 C5 After bake-out, cathode heater voltagewastapplied to raise the cathode temperature to 750 C. The cathodewas-maintained at this temperature for a period of from I 15 to 20minutesto remove surface gases. 7

structurewas then heated'by a radiofrequency generator I to a.temperature of 800 C. after which the cathode was broughtup slowly to-atemperature of 1000" C. and was maintained at this latter temperaturefor a period of 20 .minutes. After, this period the pressure in thesystem,

suddenly dropped to a lower value (from'abou't 5 10" down to about 5 X10'" millimeter of mercury) indicating substantially complete breakdownof the carbonates.

After breakdown, a potential of 50 volts was applied etc the first anodeand a potential of 1800 volts was applied to the second anode. Afterabout 20 minutes with the indicated potentials applied, the cathode drew3 amperes. According to the specification for this particular tube thiscurrent density marked activation so that the tube was then sealed offthe station.

'The oscilloscope tube so produced was operated at 850 C. with a firstanode potential of 30 volts and a second anode potential of 1000 voltsat current densities of the order of 1 ampere per square centimeter.

What is claimed is: I

1. A method of forming a cathode element comprising mixing nickel powderwith a powder material selected from the group consisting of bariumcarbonate, bariumstrontium carbonate and barium-strontium-calciumcarbonate together with an activator material, molding the resultantmixture under pressure, heating to a first temperature in the range offrom 550 C. to 650 C. in a nonoxidizing atmosphere, the heating in therange of from about 400 C. to the said first temperature being carriedout in a reducing atmosphere, further increasing the C. in anon-oxidizing atmosphere.

2. A method of forming a cathode element compris- Co'oling' from 09 I Iing mixing nickel powder with a powder material selected from the groupconsisting of barium carbonate, bariumstrontium carbonate andbarium-strontium-calcium carbonate together with an activator materialselected from the group consisting of zirconium, titanium and carbon,molding the resultant mixture under pressure, heating the pressedmixture to a temperature of about 400 C. in a non-oxidizing atmosphere,further heating from the said temperature of about 400 C. to a firsttemperature in the range of from 550 C. to 650 C. in a reducingatmosphere, further heating from the said first temperature to atemperature of at least 800 C. in an inert atmosphere, cooling to asecond temperature in the range of from 650 C. to 550 C. in an inertatmosphere, which second temperature may be the same as the said firsttemperature, further cooling from the said second temperature to atemperature of about 400 C. in a reducing atmosphere, and finallycooling to a temperature below 400 C. in a non-oxidizing atmosphere.

3. A method of forming a cathode element comprising mixing nickel powderwith a powder material selected from the group consisting of bariumcarbonate, bariumstrontium carbonate and barium-strontium-calciumcarbonate together with an activator, material selected from the groupconsisting of zirconium, titanium and carbon, molding the resultantmixture under pressure, heating to a temperature of about 400 C. in anon-oxidizing atmosphere, further heating to a first temperature in therange of from 550 C. to 650 C. in a reducing atmosphere, further heatingfrom the said first temperature to a second temperature in the range offrom about 800 C. to about 1200 C. in an inert atmosphere, cooling fromthe said second temperature to a third temperature in the range of from650 C. to 550 C. in an inert atmosphere, which third temperature may bethe same as the said first temperature, further cooling from the saidthird tem perature to a temperature of about 400 C. in a reducingatmosphere and finally cooling to a temperature below 400 C. in anon-oxidizing atmosphere.

4. A method of forming a cathode element comprising mixing nickel powderwith a powder material selected from the group consisting of bariumcarbonate, bariumstrontium carbonate and barium-strontium-calciumcarbonate together with an activator material selected from the groupconsisting of zirconium, titanium and carbon, molding the resultantmixtureunder pressure, heating from room temperature to about 400 C. inan inert atmosphere, further heating from the said temperature of about400 C. to a first temperature in the range of from 550 C. to 650 C. inhydrogen, further heating from the said first temperature to a secondtemperature in the range of from about 800 C. to about 1200 C. in anatmosphere of an inert gas selected from the group consisting ofnitrogen, helium and argon, cooling from the said second temperature toa third temperature in the range of from 550 C. to 650 C. in anatmosphere of an inert gas selected from the group consisting ofnitrogen, helium and argon, which third temperature may be the same asthe said first temperature, further cooling from the said thirdtemperature to a temperature of. about 400 C. in hydrogen and finallycooling from the said temperature of about 400 C. to room temperature inan inert atmosphere.

5. A method of forming a cathode element comprising mixing nickel powderwith a powder material selected from the group consisting of bariumcarbonate, bariumstrontium carbonate and barium-strontium-calciumcarbonate together with an activator material selected from the groupconsisting of zirconium, titanium and carbon and a binder, molding theresultant mixture under a pressure of from to tons per square inch,heating from room temperature to a first temperature in the range offrom 550 C. to 650 C. in an atmosphere of hydro gen, further heatingfrom the said first temperature to 'a temperature of about 1000 C. in anatmosphere of nitrogen, cooling from the said temperature of about 1000C. to a second temperature in the range of from 650 C. to 550 C. in anatmosphere of nitrogen, which second temperature may be the same as thesaid first temperature, and cooling from the said second temperature toroom temperature in an atmosphere of hydrogen.

6. A method of forming a cathodeelement comprising mixing nickel powderwith coprecipitated bariumstrontium carbonate together with an activatormaterial se lected from the group consisting of zirconium, titanium andcarbon and a binder material, molding the resultant mixture under apressure of from 80 tons per square inch to 100 tons per square inch,heating from room tempera ture to a first temperature in the range offrom 550 C. to 650 C. in an atmosphere of hydrogen, further heating fromthe said first temperature to a temperature of about 1000 C. in anatmosphere of nitrogen, cooling from the said temperature of about 1000"C. to a second tempera ture in the range of from 650 C. to 550 C. in anatmosphere of nitrogen, which second temperature may be the same as thesaid first temperature, and cooling from the said second temperature toroom temperature in an atmosphere of hydrogen.

7. A method of forming a cathode element comprising mixing nickelpowder-with-coprecipitated barium-strontium-calcium carbonate togetherwith an activator material selected from the group consisting ofzirconium, titanium and carbon and a binder material, molding theresultant mixture under a pressure of from 80 tons per square inch to100 tons per square inch, heating from room temperature to a firsttemperature in the range of from 550 C. to 650 C. in hydrogen, furtherheating from the said first temperature to a temperature of about 1000C. in an atmosphere of nitrogen, cooling from the said temperature ofabout 1000 C. to a second temperature in the range of from 650 C. to 550C. in an atmosphere of nitrogen, which second temperature may be thesame as the said first temperature and cooling from the said secondtemperature to roomtemperature in an atmosphere of hydrogen.

References Cited in the file of this patent UNITED STATES PATENTS1,191,552 Aeuer July 18, 1916 1,883,898 Halliwell Oct. 25, 19322,326,631 Fischer Aug. 10, 1943 2,543,439 Commes et a1 Feb. 27-, 1951

1. A METHOD OF FORMING A CATHODE ELEMENT COMPRISING MIXING NICKEL POWDERWITH A POWDER MATERIAL SELECTED FROM THE GROUP CONSISTING OF BARIUMCARBONATE, BARIUMSTRONTIUM CARBONATE AND BARIUM-STRONTIUM-CALCIUMCARBONATE TOGETHER WITH AN ACTIVATOR MATERIAL, MOLDING THE RESULTANTMIXTURE UNDER PRESSURE, HEATING TO A FIRST TEMPERATURE IN THE RANGE OFFROM 550*C. TO 650*C. IN A NONOXIDIZING ATMOSPHERE, THE HEATING IN THERANGE OF FROM ABOUT 400*C. TO THE SAID FIRST TEMPERATURE BEING CARRIEDOUT IN A REDUCING ATMOSPHERE, FURTHER INCREASING THE TEMPERATURE OF THEPRESSED MIXTURE ABOVE 650*C. AND THEN DECREASING TO A SECOND TEMPERATUREIN THE RANGE OF FROM 550*C. TO 650*C., WHICH SECOND TEMPERATURE MAY BETHE SAME AS THE SAID FIRST TEMPERATURE, THE LAST HEATING AND COOLINGSTEPS BEING CARRIED OUT IN AN ATMOSPHERE OF AN INERT GAS, FOLLOWED BYFURTHER DECREASING THE TEMPERATURE OF THE PRESSED MIXTURE FROM THE SAIDSECOND TEMPERATURE TO ABOUT 400*C. IN A REDUCING ATMOSPHERE, AND FINALLYDECREASING THE TEMPERATURE BELOW ABOUT 400* C. IN A NON-OXIDIZINGATMOSPHERE.